1. Field of the Invention
This invention relates to a rotation transmitting device which can be used e.g. as a differential of an automobile.
2. Description of the Related Art
A differential of an automobile comprises an opposed pair of side gears coupled to output shafts such as axles and pinion gears coupled to an input member coupled to the propeller shaft. If there is a difference in revolving speed between the right and left wheels, the pinion gears rotate between the side gears, thus transmitting a driving torque to the respective output shafts according to the loads on the respective wheels.
But with such a simple differential comprised merely of side gears and pinion gears meshing with each other, if one of the wheels should get off the track or move onto a frozen or snow-covered surface so that the load thereon should drop sharply, the turning torque is distrubuted only to the idling un-loaded wheel, whereas torque transmission is completely shut off to the wheel still gripping the road surface.
In order to cope with this problem, many devices have been proposed which limit the differential function of a differential. As typical examples, there are known a differential having a viscous coupling adapted to distribute the driving torque to the wheel under high load making use of the shearing resistance of a high-viscosity fluid and a differential having a plurality of friction plates and elastic members adapted to retard the movement of the side gears and thus to transmit the driving torque to both output shafts.
But in the former arrangement using a viscous coupling, while the difference in revolving speed between the output shafts is small, the shearing resistance of the high-viscosity fluid is small, so that no sufficient driving torque is obtainable. If attempt is made to increase the shearing resistance while the revolving speed is low, a dragging torque tends to appear while the car is turning a tight corner at a low speed.
On the other hand, in the latter arrangement using friction plates, the internal structure of the differential tends to be complicated and heavy. When the car turns a tight corner with the friction plates in operation, a dragging torque will be produced by the frictional force on the side gears.